A new process for the enrichment of nickel in

ISBN: 8I-87053.53-4
Processing of Fines (2)
F-dc I' Bhattarhai via. R Singh and N. G. Gosu'(tnli
0 NMI. J w:.sltrdpui-A3/ 007. India. 200(1, pp. 307-316
A new process for the enrichment of nickel in
Sukinda chromite overburden ore
S. BHATTACHARJEE, P. DASGUPTA, D. KAR. R. N. BHATTACHARJEE,
S. GHOSAL and A. R. PAUL.
National tiletcnlhu,Gical L<ahoratorv, Jurnshedput; 83/007. India
ABSTRACT
Chromite Over Burden (COB) ore front Sukinda valley, Orissa is too
lean in nickel concentration to he exploited for the extr action of nickel.
The average nickel concentration in the COB ore lies in the range of
0.4-0.6%. An acid leaching route at atmospheric prsssure has been
developed for screening out silica as insoluble. Metal values in the
leach liquor are retrieved as hydroxides by appropriate manoeuvring
of pH. The metal hydroxides are converted to corresponding oxides bi
calcining at 900°C. The resultant nickel content in the mixed oxide has
been consistently f nand to be 1.6% and above. This product may be
directly used as a starting material for the preparation of ferm-nickel.
Key words : Chi unite overburden ore. Nickel. Acid leaching, Ferro-nickel
INTRODUCTION
Sukinda valley of Orissa is well known for its rich reserve of chromite ore.
Mining of chromite ore generates overburden from the upper layer, which is
typical of its own. This overburden, which is also known as Chromite Over
Burden (COB) ore, is the only indigenous source of nickel in the country.
The average nickel concentration in the COB ore of Sukinda lies in the range
of 0.4-0.7%. The existing deposit of COB ore in the Sukinda valley has been
estimated to he around 140 million tonnes"'. This makes COB ore a highly
prospective source of nickel, especially in view of the fact that nickel is a strategic metal and entire nickel demand of our country is met through import.
Two radically different types of nickel ores, namely, sulphide ores and oxide
ores have been recognised. Sulphide ores occur as layers and lenses of mafic and
ultramatic rocks while oxide ores occur due to the weathering and laterization of
the parent metal hearing rock'''. COB ore of Sukinda valley comes under the
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S. BIL-tTLtCIIARJEL• el. al.
second category. the oxides. The primary mineral phases in the COB ore are
quart /. goethite and limonite in association with other minor phases like magnetite. hematite. chroniite etc.
Early exploits of nickel were mostly from sulphide ores except till 1943 when
Freeport Sulphur Company for the first time tried to harness the lateritic oxide
ores for the purpose of extracting nickel'. It was a daunting task at that time
since the nickel content in the starting material was even less than 1.5C7c. Caron's
process of ore pre-reduction followed by ammonia-ammonium carbonate leaching was adopted. Nickel''' was extracted as NiCO1/ Ni(OH),. Freeport's Enterprise was, however , aborted because of technical non -viability.
Parallel developments were made on acid leaching of the lateritic oxide ore.
Sulphation of oxide ores was attempted by several workers. Both high and atmospheric pressure leaching routes were attempted. Freeport Sulphur Company put
up another plant at Moa Bay, Cuba in which lateritic oxide ores with it nickel
content of about I.31; was subjected to high pressure sulphuric acid leaching"
Extraction of nickel was excellent and almost selective thus requiring less amount
of sulphuric acid. The process was, however, plagued with severe corrosion
problem.
Atmospheric pressure acid leaching of nickeliferrous laterites received popular attention because of a number of apparent advantages that include ease of
operation , less corrosion and low energy consumption . However, atmospheric
pressure acid leaching has the disadvantage of being non-selective with high acid
consumption . Acid leaching of lateritic ores under atmospheric pressure was
attempted by Rice and Strong4' in the early seventies but did not receive much
favour at that time. Decade of eighties saw a spurt of research in this area. A
number of inorganic and organic acids were investigated as probable lixiviants
for COB ores'I. Among the inorganic acids, rate of extraction was highest for
HCI. Pre-roasting of the COB ore at about 360°C decomposes the goethite phase
to hematite. Shukla and Das'"I studied the effect of this transformation on the
leaching behaviour of Sukinda COB ore. Shukla and co-workers further carried
out a comparative study on the leaching of COB ore in HCI and H,SO41'. Under
similar reaction conditions HCI produce better extraction than H,SO4. Pre-roasting of the ore increased the extractability. Panagiotopoulos and co-workers worked
on the atmospheric pressure sulphuric acid leaching of serpentinic"I and towgrade hematitic laterites'''. Acid leaching of Australian nickeliferrous laterites at
atmospheric pressure was carried out by Canterford with reference to leaching
temperature. acid strength. ore mineralogy, pre-roasting temperature etc.'"'"
Present paper describes a new process for the chemical enrichment of nickel
content in high silica Sukinda COB ore. The process is based on the dissolution
of the COB ore through atmospheric acid leaching and retrieving the metal
i(I`i
S. /1114 ACHAKJE
el. ul,
values through precipitation as hydroxide. A nickel value of 1.6'7( or above
qualifies the final product as a starting material for producing ferro-nickel.
Exl'F.RIMiENTAL
Digestion experiments for optimising the leaching parameters were carried
out in a three-necked Borosil flask under refluxing condition and constant agitation . In each hatch 500 g of the COB ore was used. The extractability of
different sieve fractions was performed in open beakers. I g of each fraction was
subjected to acid digestion in the proportion. 5 ml HCI. 5 ml HNO„ 10 ml H,SO4
and 20 ml H,O kept over a hot plate for a period of two hours with occasional
stirring. Total analyses of the sieved fractions were carried out by dissolving I
g of each fraction in HCI HNO HCIO, tri-acid mixture using standard protocols.
SiO, was determined by standard HF digestion of the residue. All the metal
analyses were performed using GBC 908 AA Atomic Absorption Spectrometer.
All the reagents used were of AR grade with 18MS2 ASTM Grade I distilled
water was used for making the solutions.
RESULT's AND DISCUSSION
Table I gives the complete chemical analysis of the COB ore used in the
present study. As a prerequisite for the acid leaching of COB ore. The ore must
be ground to a fine site to make more and more surface area available for acid
attack. However, grinding of the ore has a cost, which increases sharply with
finer size requirement. This warrants an optimisation, as the final techno-economics of the process should be influenced by the grinding cost.
To arrive at an optimum particle size following exercise was carried out. I
kg ore of -10 mesh ore was passed through sieves of different sizes and their
percentages were calculated. Percentages of different fractions as recovered
from 1 kg of -10 mesh ore are shown in Table 2.
It may be observed from Table 2 that -10 mesh COB ore was a well distributed matrix. All the sieve fraction collected were analysed for their principal
components. namely, LOI, SiO„ Fe, Ni, Co, Cr and Mn and the results have been
shown in Table 3. It is evident from Table 3 that grinding to finer sizes leads to
the liberation of metal values and silica percentage goes down considerably.
There has been an enrichment of all the metals in -2(X) mesh fraction. However,
it was also interesting to find that metal values did not vary significantly in
fractions B. C and D. Thus, it may be inferred that grinding upto -44 mesh size
should he sufficient as far as metal liberations are concerned.
To look into this observation more critically leaching experiments were carried out with all the sieved fractions under similar reaction conditions. I g of
each fraction was digested with 5 ml HCI. 5 ml HNO1 and 10 nil H,SO4 under
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S. BHA7T1 CHARJEE er. al.
identical conditions. The leach liquor in each case was analysed for the metal
values and the percentage extractability was calculated on the basis of analytical
data for each metal in each fraction given in Table 3. The percentage extractahilitv of each metal in each fraction is given in Table 4.
Table I : Complete chemical analy sis of the COB ore
Ele men (/Radical
Content (Ir
LOI
5.15
SiO,
55.39
Fe,O.
34.41
NiO
0.93
CoO
0.05
Cr,O1
2.01
MnO
0.57
ZnO
0.01
Na,O
0.16
CaO
0.35
MgO
0.74
"!'able 2 : Sieve analysis of -10 mesh COB ore
Label Sieve fraction, mesh
Weight, a
Weight, %
A
-200
165
16.5
13
+200 to -10(I
265
26.5
C
+I M to -60
186
18.6
D
+60 to -44
145
14.5
E
+44
214
21.4
Loss
25
Total
10(H)
310
.
2 5
10(1
S. BHA77A(71t XiL7i et. ul.
Table 3 : Chemical analyses of different sieve fractions
Element (17O
Sieve fractions
A
B
C
D
LOI
7.73
5.29
5.59
5.76
5.7
SiO,
43.98
48.62
55.32
58.92
58.90
Ni
1.1
0.756
0.747
0.766
0.701
Co
0.062
0.031
0.036
0.042
0.051
Cr
1.27
1.21
1.07
1.33
0.96
Fe
32.48
20.16
19.04
20.16
20.16
MI)
0.65
0 .43
0.46
0.5
0.47
E
Data in Table 4 make some interesting observations. On increasing the fineness of the ore particles, one expected a higher extraction. However, for all the
metals reported, fraction B had shown maximum extractability though fraction
A was of highest fineness . The leachant combination of HCI, HNO, H,SO4 and
water was adhoc in nature and no attempt was made at this stage to optimise the
leachant combination. The objective of this study was to find out the metal
extractability of different sieve fractions under similar reaction conditions. It was
not readily explainable why the extractability was less in fraction A. It was
further interesting to note that metal extractability of fractions B, C and D did
not differ much . This strengthens our earlier contention that a grinding upto -44
mesh was sufficient for the leaching of the COB ore and there was no need to
go for expensive finer grindings.
Table 4 : Percentage extractability of metals in each .sieved fraction
Element (°k)
% Extractability in fraction
A
B
C
D
E
Ni
68.20
75.39
70.95
71.80
78.45
Co
46.77
61.29
55.55
61.90
54.90
Cr
19.68
20.66
23.27
22.93
29.17
Fe
83.43
91.07
90.02
90.57
77.77
Mn
58.46
66.51
63.04
66.01
61.96
Process for Nickel Enrichment
COB ore of Sukinda valley is a high silica matrix. Present nickel enrichment
process consists of primarily three stages. In the first stage the ore is subjected
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S. BHA7TAC II AR/hI. et. at
to acid digestion which soluhilizes the metal values and screens out the silica. In
the second stage the leach liquor that consists of salts of Ni. Co. Cr. Fe and other
trace metals is neutralised with an appropriate alkali to precipitate the hydroxides
of Ni, Co. Cr and Fe. In the third stage the metal hydroxides are calcined at a
higher temperature to achieve the desired grade of nickel. Fig. I gives the schematic diagram of the proposed enrichment scheme.
Stage I : Digestion with acid/ Combination of acids, HCI, HNO and H,SO4.
Stage lI Neutralisation with alkali. Na,CO,. NaOH.
Stage III : Calcination at 900°C
Raw nickel ore
Ni : 0.4-0.7%
I
Digestion
Leach liquor of N
Co, Cr and Fe
Neutralisation I 11
III
Oxides of Ni, Cr
Co and Fe Calcination
9(X)11C
Y
Hydroxides of Ni
Co. Cr and Fe
Fi'.l : Schematic diagram of the nickel enrichment schetne
Optimisation of* leaching parameters :
Leaching parameters for the optimum leaching of nickel from Sukinda COB
ore were identified as, HCI, HNO, and H,SO4 concentrations, reaction temperature, amount of water added and digestion time. A large number of experiments
(about 40) were carried out in the laboratory to optimise these parameters. The
parameters as optimised are given below.
Under the optimised conditions the extraction of nickel from the COB ore
was slightly more than 804.
Neutralisation of the leach liquor
The leach liquor contains salts of Co, Fe, Cr and traces of Al, Mn and Zn.
These metals were precipitated using an appropriate neutralising agent. Both
Na,CO, and NaOH were used. However, use of Na,CO, is recommended as
washing the precipitate. which is a very crucial step for obtaining the product
grade, is easier with Na2CO, than NaOH. NH4OH cannot he used as part of the
nickel and cobalt get solubilised and the product yield goes down.
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S. BHA774CH1RJ1: F. et. a!.
Table 4 : Optimised parameters with reference to the extraction of Nickel
Parameters
Optimised values
COB ore
500 g
Fineness
-44 mesh
HCI concentration
50 ml 12N
HNO, concentration
50 nil 14N
H,SO4 Concentration
100 ml 36N
H,O
500 ml
Digestion time
3 hours
Temperature
100°C
Calcination of the hydroxides :
Metal hydroxides produced during the neutralisation process must be converted into the corresponding oxides to achieve the desired grade of nickel.
Though it has been shown in the schematic diagram that the hydroxides were
calcined at a temperature of 900°C, a thorough study was carried out to find out
the optimum calcination temperature . The results of calcination temperature
optimisation for a typical metal hydroxide have been given in Table 5. It is
evident from Table 5 that calcination of the hydroxides is a need - based requirement . If the starting material has a higher grade of nickel it may not be at all
necessary to calcine the product if the desired nickel percentage is 1.6% and a
mere demoisturisation at 110°C should he sufficient, as may be seen from Table
5. However, most of the COB ores are very poor with respect to nickel concentration and calcination at a higher temperature appears to be a necessary step.
This may be mentioned again at this stage that washing of the metal hydroxide
precipitate is an extremely crucial step for achieving the product purity. Presence
of sodium salts has a tendency to deteriorate the product purity to a great extent.
Final product :
The final product after the calcination was essentially a mixed oxide of Fe,
Ni, Cr, Co and other trace elements like, Al, Mn , Zn, Na. Ca etc . Complete
analysis of a typical final product is given in Table 6.
X-ray diffraction of the final product ( not shown here ) indicated that it consisted of two phases, (-Fe,O, and nickel ferrite, NiFe,O4. It was observed that the
final product was weakly magnetic in nature which supported the formation of
weakly magnetic NiFe,O4 phase . It could he possible that the final product consisted of a magnetic and a non magnetic phase. (-Fe,O, constituted the
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S. R1141"IACIIAR.lE1:' et. al.
fable /iiiect o/ heat treatment on the metal hYdroxides
obtained front leach liquor
Sample
Temperature "C
Nike/
Co .'
Fe. %
OAR,
Ito
1.68
0.063
56.0
O„R,
1.66
0.077
O „R ,
300
500
1.81
0.084
58.2
63.8
0„R ,
700
1 . 91
0.088
67.2
O„R 1
900
[95
0.086
69.4
Table 6 : Complete analysis of the final product
Element/Radical
Content %
1.01
0.11
SiO,
0.13
Fe2O
992.12
NiO
2.54
Cr,O
11.75
CoO
0.14
AI,O,
1.21
MnO
1.42
ZnO
0.38
Na,O
0.67
nonmagnetic fraction while NiFe,O, made for the magnetic fraction . This opened
up the possibility of further nickel enrichment by separating the magnetic and
non-magnetic fraction through magnetic separation. A study was carried out to
ascertain the extent of NiFe,O, formation through measurement of saturation
magnetisation . It was calculated from chemical analysis that in a typical final
product. assuming all the nickel were converted into NiFc ,O,. the percentage of
NiFe,O, would have been 9.37'. The same product yielded a saturation
magnetisation value of 4 . 5 emu/g . Considering the standard saturation
magnetisation value of NiFe,O , 50 emu /g, the amount of NiFe , O, in the product
should have been 9 .049k. The nearness of the NiFe,O, percentage data obtained
from two completely independent methods vindicated our contention that the
entire nickel was converted into NiFe _ O, and the final product consisted of a
magnetic and a non - magnetic phase . Thus, in principle it is possible to enrich the
nickel content further by separating the magnetic phase
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S. 811A1T1 ('11,4RIlili ri. tit
Merits and strengths of the present process :
The present process is attractive because it is essentially a low energy consumption process. The existing nickel exploitations from lateritic oxides, whether
by acidic or alkaline route, are all energy intensive processes. Also these processes need to crush the ore to a very fine size (about -200 mesh). The present
process can handle ores of -44 mesh size quite easily. The leaching temperature
is only 100-110°C. Calcination at a higher temperature is only a need-based
operation and will depend on the quality of the starting material and percentage
extraction of nickel. Even if calcination is required, a much smaller mass compared to the starting material needs to be handled and going upto 900°C may not
be necessary. Reagents used in the present process are inexpensive and known
to have no toxic effects on the operator provided general laboratory safety norms
are adhered to. The process is marked with operational simplicity involving
essentially digestion, precipitation and filtration. The process will generate valuable by-products, which has a steady market within the country. The process
does not envisage any waste and effluent disposal problem thus making it environment friendly.
CONCLUSION
Conclusions of the present study may he listed as follows :
1. COB ore of Sukinda valley may he leached with an appropriate combination of HCI. HNO,. H2SO, and water. Extractability of nickel is more
than 80(7.
2. An ore size of -44 mesh is sufficient for acid digestion.
3. Neutralisation of the leach liquor is carried out with Na,CO, and NaOH.
4. Washing of metal hydroxide precipitate is extremely crucial for achieving
the desired product grade.
5. Calcination of the hydroxide precipitate at higher temperature enhances
the nickel content in the final product. However, calcination is not a
necessary step and will depend on the quality of the starting COB ore and
extractability of nickel.
6. The process is not energy intensive and generates valuable by-products
that have ready market in the country.
7. The process is environment friendly and does not encounter waste and
effluent disposal problems.
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S. B1GI1T.-1C/I.4R.IEI: et. of
ACKNOW l,FJK;F:^1ENT
The authors wish to thank the Director. NML. Jamshedpur, for his kind
permission to publish this work.
Rh:FI:RENCha
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